This invention relates to the exploitation of hydrocarbon-bearing earth formations, and, more particularly, to a system and method for the in situ heating processing of hydrocarbon-bearing earth formations such as oil shale, tar sands, coal, heavy oil, and other bituminous or viscous petroliferous deposits. The present subject matter is related to subject matter set forth in the copending U.S. application Ser. No. 828,621, of Jack Bridges, Allen Taflove and Richard Snow, filed of even date herewith and assigned to the same assignee as the present application.
Large scale commercial exploitation of certain hydrocarbon-bearing resources, available in huge deposits on the North American continent, has been impeded by a number of problems, especially cost of extraction and environmental impact. The United States has tremendous coal resourses, but deep mining techniques are hazardous and leave a large percentage of the deposits in the earth. Strip mining of coal involves environmental damage or expensive reclamation. Oil shale is also plentiful in the United States, by the cost of useful fuel recovery has been generally noncompetitive. The same is true for tar sands which occur in vast amounts in Western Canada. Also, heavy or viscous oil is left untapped, due to the extra cost of extraction, when a conventional oil well is produced.
Materials such as oil shale, tar sands, and coal are amenable to heat processing to produce gases and hydrocarboneous liquids. Generally, the heat develops the porosity, permeability and/or mobility necessary for recovery. Oil shale is a sedimentary rock which, upon pyrolysis or distillation, yields a condensable liquid, referred to as a shale oil, and non-condensable gaseous hydrocarbons. The condensable liquid may be refined into products which resemble petroleum products. Oil sand is an erratic mixture of sand, water and bitumen with the bitumen typically present as a film around water-enveloped sand particles. Using various types of heat processing the bitumen can, with difficulty, be separated. Also, as is well known, coal gas and other useful products can be obtained from coal using heat processing.
In the destructive distillation of oil shale or other solid or semi-solid hydrocarbonaceous materials, the solid material is heated to an appropriate temperature and the emitted products are recovered. This appears a simple enough goal but, in practice, the limited efficiency of the process has prevented achievement of large scale commercial application. Regarding oil shale, for example, there is no presently acceptable economical way to extract the hydrocarbon constituents. The desired organic constituent, known as kerogen, constitutes a relatively small percentage of the bulk shale material, so very large volumes of shale need to be heated to elevated temperatures in order to yield relatively small amounts of useful end products. The handling of the large amounts of material is, in itself, a problem, as is the disposal of wastes. Also, substantial energy is needed to heat the shale, and the efficiency of the heating process and the need for relatively uniform and rapid heating have been limiting factors on success. In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is again relatively large, since bitumen typically constitutes only about ten percent of the total, by weight. Material handling of tar sands is particularly difficult even under the best of conditions, and the problems of waste disposal are, of course, present here too.
There have been a number of prior proposals set forth for the extraction of useful fuels from oil shales and tar sands in situ but, for various reasons, none has gained commercial acceptance. One category of such techniques utilizes partial combustion of the hydrocarbonaceous deposits, but these techniques have generally suffered one or more of the following disadvantages: lack of precise control of the combustion, environmental pollution resulting from disposing of combustion products, and general inefficiency resulting from undesired combustion of the resource.
Another category of proposed in situ extraction techniques would utilize electrical energy for the heating of the formations. For example, in the U.S. Pat. No. 2,634,961 there is described a technique wherein electrical heating elements are imbedded in pipes and the pipes are then inserted in an array of boreholes in oil shale. The pipes are heated to a relatively high temperature and eventually the heat conducts through the oil shale to achieve a pyrolysis thereof. Since oil shale is not a good conductor of heat, this technique is problematic in that the pipes must be heated to a considerably higher temperature than the temperature required for pyrolysis in order to avoid inordinately long processing times. However, overheating of some of the oil shale is inefficient in that it wastes input electrical energy, and may undesirably carbonize organic matter and decompose the rock matrix, thereby limiting the yield. Further electrical in situ techniques have been termed as "ohmic ground heating" or "electrothermic" processes wherein the electric conductivity of the formations is relied upon to carry an electric current as between electrodes placed in separated boreholes. An example of this type of technique, as applied to tar sands, is described in U.S. Pat. No. 3,848,671. A problem with this technique is that the formations under consideration are generally not sufficiently conductive to facilitate the establishment of efficient uniform heating currents. Variations of the electrothermic techniques are known as "electrolinking", "electrocarbonization", and "electrogasification" (see, for example, U.S. Pat. No. 2,795,279). In electrolinking or electrocarbonization, electric heating is again achieved via the inherent conductivity of the fuel bed. The electric current is applied such that a thin narrow fracture path is formed between the electrodes. Along this fracture path, pyrolyzed carbon forms a more highly conducting link between the boreholes in which the electrodes are implanted. Current is then passed through this link to cause electrical heating of the surrounding formations. In the electrogasification process, electrical heating through the formations is performed simultaneously with a blast of air or steam. Generally, the just described techniques are limited in that only relatively narrow filament-like heating paths are formed between the electrodes. Since the formations are usually not particularly good conductors of heat, only non-uniform heating is generally achieved. The process tends to be slow and requires temperatures near the heating link which are substantially higher than the desired pyrolyzing temperatures, with the attendant inefficiencies previously described.
Another approach to in situ processing has been termed "electrofracturing". In one variation of this technique, described in U.S. Pat. No. 3,103,975, conduction through electrodes implanted in the formations is again utilized, the heating being intended, for example, to increase the size of fractures in a mineral bed. In another version, disclosed in U.S. Pat. No. 3,696,866, electricity is used to fracture a shale formation and a thin viscous molten fluid core is formed in the fracture. This core is then forced to flow out of the shale by injecting high pressured gas in one of the well bores in which an electrode is implanted, thereby establishing an open retorting channel.
In general, the above described techniques are limited by the relatively low thermal and electrical conductivity of the bulk formations of interest. While individual conductive paths through the formations can be established, heat does not radiate at useful rates from these paths, and efficient heating of the overall bulk is difficult to achieve.
A further proposed electrical in situ approach would employ a set of arrays of dipole antennas located in a plastic or other dielectric casing in a formation, such as a tar sand formation. A VHF or UHF power source would energize the antennas and cause radiating fields to be emitted therefrom. However, at these frequencies, and considering the electrical properties of the formations, the field intensity drops rapidly as a function of distance away from the antennas. Therefore, once again, non-uniform heating would result in the need for inefficient overheating of portions of the formations in order to obtain at least minimum average heating of the bulk of the formations.
A still further proposed scheme would utilize in situ electrical induction heating of formations. Again, the inherent (although limited) conduction ability of the formations is relied upon. In particular, secondary induction heating currents are induced in the formations by forming an underground toroidal induction coil and passing electrical current through the turns of the coil. The underground toroid is formed by drilling vertical and horizontal boreholes and conductors are threaded through the boreholes to form the turns of the toroid. It has been noted, however, that as the formations are heated and water vapors are removed from it, the formations become more resistive, and greater currents are required to provide the desired heating.
The above described techniques are limited by either or both of the relatively low thermal and electrical conductivity of the bulk formations of interest. Electrical techniques utilized for injecting heat energy into the formations have suffered from limitations given rise to by the relatively low electrical conductivity of the bulk formations. In situ electrical techniques appear well capable of injecting heat energy into the formations along individual conductive paths or around individual electrodes, but this leads to non-uniform heating of the bulk formations. The relatively low thermal conductivity of the formations then comes into play as a limiting factor in attaining a relatively uniformly heated bulk volume. The inefficiencies resulting from non-uniform heating have tended to render existing techniques slow and inefficient.
It is an object of the present invention to provide in situ heat processing of hydrocarbonaceous earth formations utilizing electrical excitation means, in such a manner that substantially uniform heating of a particular bulk volume of the formations is efficiently achieved.
Further objects of the present invention are to provide a system and method for efficiently heat processing relatively large blocks of hydrocarbonaceous earth formations with a minimum of adverse environmental impact and for yielding a high net energy ratio of energy recovered to energy expended.